1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film by using a laser beam (hereinafter referred to as laser annealing). The invention also relates to a method of manufacturing a semiconductor which includes the laser annealing method as one step. Incidentally, the term “semiconductor device” used herein generally includes an electro-optical devices such as liquid crystal display devices and light emitting devices as well as electronic equipment including the electro-optical devices as constituent parts.
2. Background Art
In recent years, a wide range of researches have been made as to the art of applying laser annealing to a semiconductor film formed on an insulating substrate such as a glass substrate to crystallize the semiconductor film or to improve the crystallinity thereof. Silicon is widely used for such a semiconductor film. In the present specification, means for crystallizing a semiconductor film by a laser beam to obtain a crystalline semiconductor film is referred to as laser crystallization. In addition, the crystalline semiconductor film indicates a semiconductor film in which a crystalline region is existed in the present specification.
As compared with synthetic quartz glass substrates which have heretofore widely been used, glass substrates have the advantages of being inexpensive and rich in workability and of facilitating fabrication of large-area substrates. This is the reason why a wide range of researches have been made. The reason why lasers are preferentially used for crystallization of glass substrates is that the melting points of glass substrates are low. Lasers can give high energy to semiconductor films without increasing the temperatures of substrates to a great extent. In addition, lasers are remarkably high in throughput compared to heating means using electric heating furnaces.
Since a crystalline semiconductor film formed by the application of laser annealing has high mobility, the crystalline silicon film is used to form thin film transistors (TFTs). The thin film transistors are widely used in a monolithic type of liquid crystal electro-optical device in which TFTs for pixel driving and TFTs for driver circuits are fabricated on one glass substrate.
A method of effecting laser annealing by forming a high power pulsed laser beam such as an excimer laser beam, by an optical system, into a laser beam which becomes a spot of several cm square or a linear shape of length 10 cm or more at an irradiation plane, and scanning the laser beam (or relatively moving a position irradiated with the laser beam with respect to an irradiation plane) has preferentially been used because the method is high in productivity and superior in industrial terms.
Particularly when a linear laser beam is used, high productivity can be realized because the entire irradiation plane can be irradiated with the linear laser beam by scanning in only directions perpendicular to the lengthwise direction of the linear laser beam, unlike the case where a spot-shaped laser beam is used which needs to be scanned in forward, rearward, rightward and leftward directions. The reason why the linear laser beam is scanned in the direction perpendicular to the lengthwise thereof direction is that the lengthwise direction is the most efficient scanning. Because of this high productivity, in the laser annealing method, the use of a linear laser beam into which a pulse oscillation excimer laser beam is formed by an appropriate optical system is presently becoming one of leading manufacturing techniques for a liquid crystal display devices using TFTs.
However, there are cases in which the film quality of a crystalline semiconductor film, obtained by a crystallization method employing laser light, falls. That is, if laser light is irradiated to a semiconductor film, then the semiconductor film instantaneously melts and locally expands, and distortion develops in the crystalline semiconductor film in order to relieve internal stresses that develop due to the expansion.
Further, a high energy can be imparted to the semiconductor film without much of a rise in the temperature of a substrate when using a crystallization method employing irradiation of laser light. A steep temperature gradient therefore develops between the substrate and the semiconductor film, and the semiconductor film distorts due to tensile stresses.
If distortion exists in the semiconductor film in an insulating gate semiconductor device, then potential barriers and trap levels are formed due to the distortion, and therefore the interface level between an active layer and a gate insulating film becomes high. Furthermore, an electric field is not applied uniformly if distortion exists, and this becomes a cause of operation failures of the semiconductor device. In addition, distortion of the surface of the semiconductor film damages the levelness of the gate insulating film, deposited by sputtering or CVD, and causes reliability to drop due to occurrences of insulating defects and the like. One important factor in determining electric field effect mobility of a TFT is a surface scattering effect. The degree of levelness of the interface between the active layer and the gate insulating film of the TFT exerts a large influence on the electric field effect mobility. High electric field effect mobility is obtained, without influence caused by scattering, the more level the interface becomes. Distortion of crystalline semiconductor films thus influences all of the TFT properties, and even a yield changes.